Recently, attention is directed to application of a photonic crystal to an optical integrated circuit. For example, a device for changing a spot size of light guided by a light waveguide is disclosed in Patent Document 1, while an optical control device that has a dispersion controlling effect is disclosed in Patent Document 2.
Patent Document 1: JP Patent Kokai JP-A-2003-240985
Patent Document 2: JP Patent Kokai JP-A-2005-274844
The disclosures of the Patent Documents 1 and 2 are to be incorporated herein by reference thereto. The following is an analysis on the related art according to the present invention. Nowadays, there is raised a demand for a technique for implementing an integrated circuit of optical components, such as a transistor integrated circuit. In the current state of the art, an optical fiber, as a waveguide, and a variety of optical components, such as a light switch, a wavelength filter or a 3 dB coupler, as individual optical components, are interconnected to form an optical circuit. If these optical components could be integrated on a small chip, it would be possible to reduce the circuit volume, power consumption and the manufacturing cost significantly.
A large variety of techniques for implementing optical integrated circuits have so far been developed. In particular, photonic crystals are stirring up notice as being a technique having a potential capability of reducing the size and the power consumption per unit performance of an optical device fabricated on a substrate to one hundredth or even to one ten-thousandth.
In a broad sense of the term, a photonic crystal is a generic name of structures the refractive index of which is caused to vary periodically. Although photonic crystals are used in general for an electromagnetic wave, they are so named because they were inherently devised for optical use and also because they were endowed with a periodic structure like that of a crystal.
The photonic crystals exhibit a wide variety of optical features, based on the periodicity of their refractive index. Their most representative feature is the photonic band gap (PBG). As periodic changes in the refractive index of a photonic crystal are increased, the light of a particular frequency band (or a wavelength band) ceases to be able to be propagated through the photonic crystal. If the light frequency and the wave number (, or the amplitude of wave vector in a certain direction) of light are assigned to the ordinate and abscissa axes, respectively, and the relationship between the frequency and the wave number of light propagated through the photonic crystal is plotted, we obtain a diagram termed a dispersion relationship diagram or a photonic band diagram. In this photonic band diagram, the frequency range in which plots are present in succession and distributed as a curve is termed a band. The light that cannot be propagated through the photonic crystal has a frequency that is located intermediate between neighboring bands, that is, in a gap between the neighboring bands. The forbidden band, through which light cannot be propagated, is termed a photonic band gap (PBG).
If there is a defect in the photonic crystal that might disturb periodicity of the refractive index distribution of the photonic crystal, the light at a frequency falling within the PBG is confined in the vicinity of the defect. In this case, only the light at a particular frequency corresponding to the size of the defect can be confined, with the photonic crystal functioning as an optical resonator. Hence, the photonic crystal may be used as a frequency filter or a wavelength filter.
If miniscule defects are arrayed within the photonic crystal in succession in a row to form a line defect, even the light having the frequency within the PBG can be propagated along the defect, while the light is confined in the vicinity of the line defect. It may thus be seen that the line defect of the photonic crystal plays the role of a waveguide. This waveguide may be called a line-defect waveguide.
If a filter and a waveguide can be implemented, a light modulator or a light switch may be constructed by solely the waveguide or by the waveguide and the filter in combination.
It is thus possible to arrange principal optical functional components in a photonic crystal and to interconnect the components to construct an optical circuit. Hence, the photonic crystal is expected to be used as a platform for the optical integrated circuit.
From the perspective of manufacture, a photonic crystal desirably has a two-dimensional periodicity. If it is attempted to utilize the effect of the PBG, such as light confining effect, the photonic crystal must have three-dimensional periodicity in three perpendicular directions of x, y and z. However, fabrication process for a three-dimensional periodic structure is complex and hence is expensive in manufacture. For this reason, a two-dimensional photonic crystal, having a two-dimensional periodicity in a substrate surface and having a finite size along the direction perpendicular to the substrate surface, that is, along the direction of substrate thickness, is often used. In such case, light confinement along its thickness in a line-defect waveguide or in a point defect resonator is secured not by the PGG effect, but rather by total internal reflection caused by difference in the refractive index.
Strictly speaking, the optical characteristic of the two-dimensional photonic crystal having a finite thickness is not perfectly coincident with that of a two-dimensional photonic crystal having an infinite thickness, that is, a crystal that is uniform along its thickness. However, if the refractive index distribution along the direction of thickness of the two-dimensional photonic crystal having a finite thickness exhibits mirror symmetry (, or reflection symmetry) within a structural region where light is propagated, the optical characteristic of the two-dimensional photonic crystal having the finite thickness is roughly coincident with that of the two-dimensional photonic crystal having an infinite thickness. Prediction of the operation of a device of a photonic crystal with infinite thickness, that is, a device of the two-dimensional photonic crystal uniform along its thickness, is easier than that for which a finite thickness is taken into account. Hence, device implementation may be facilitated by exploiting the two-dimensional photonic crystal having refractive index distribution with the mirror symmetry.
Among a number of examples of a concrete structure of the two-dimensional photonic crystal having a finite thickness, thus far implemented, there are a hole-type photonic crystal and a pillar-type photonic crystal. In particular, a line-defect waveguide in the latter type crystal, that is, the pillar-type photonic crystal, is superior inter alia in the wave guiding characteristic.
FIG. 1 depicts a perspective view of a typical structure of a pillar-type photonic crystal with a finite thickness. Referring to FIG. 1, a multiplicity of pillars 2 with a finite height, made of a dielectric material, is arrayed in a square lattice pattern in a background medium 1. The dielectric material of the pillars has a dielectric constant higher than that of the background medium. By the way, the hole-type photonic crystal has a structure such as, in the structure shown in FIG. 1, the background medium 1 is made of a high dielectric constant material and the pillars 2 (columnar-shaped structures) are made of a low dielectric constant material. In a columnar-type square-lattice photonic crystal with a finite thickness, a line-defect waveguide can be formed by providing a row of dielectric pillars 3 as a line defect, a diameter of the pillars in the row, for example, being smaller than that of the pillars that make up otherwise perfect original crystal. In this case, the row of the columns of the line defect, is equivalent to a core in a waveguide that confines light by total internal reflection, such as an optical fiber, while the lattice(s) formed by pillars, disposed on each side of the row of pillars of the line defect, is(are) equivalent to a cladding.
In general, dielectric pillars in a photonic crystal are not limited to (circular) columns, and can be in any shape. It should be noted that a given dielectric pillar being ‘thicker’ or ‘thinner’ than other pillars means that the pillar in question is respectively larger or smaller in cross-sectional area than the others. In the present specification, the ‘dielectric pillar’ is defined as a pillar that has a dielectric constant, and encompasses air or vacuum, too.
The line-defect waveguide features a small group velocity and hence may be used as an optical delay element. In addition, the group velocity being small increases time of interaction between the guided light and the crystal material, as a result of which the effect of the interaction can be enhanced to a sufficient degree even if the waveguide is short. In other words, the effect of interaction per unit length is increased. Thus, with the line-defect waveguide, non-linear effects can efficiently be derived.
As for the line-defect waveguide of the pillar-type square-lattice photonic crystal, the group velocity of the guided light can be as small as one-twentieth to one-hundredth the speed of light in vacuum, as an example. Thus, even a short waveguide, can provide long delay time and hence a strong interaction with the material.
However, with this kind of waveguide that gives small group velocity of light, it is often experienced that the group velocity of the guided light is varied according to the light wavelength. Such wavelength-dependent variation in the group velocity is termed the ‘group velocity dispersion’. The line-defect waveguide of the columnar square-lattice photonic crystal does exhibit the group velocity dispersion. Thus, if line-defect waveguide of the columnar square-lattice photonic crystal guides an optical signal spanning a non-negligible range of wavelength, such as ultra-high speed optical signal, a problem may be that the signal waveform becomes collapsed (or deformed) after passing through the waveguide.
Patent Document 2 discloses an optical control device that compensates for the group velocity dispersion. However, the optical control device disclosed therein is provided with a structure formed by directly interconnecting a plurality of photonic crystals having different values of group velocity dispersion, raising a problem that a loss due to reflection at a connection interface may be produced.
It is therefore an object of the present invention to provide an optical delay element making use of a line-defect waveguide of a square-lattice photonic crystal in which, by reducing the effect of the group velocity dispersion, long delay time and the low group velocity dispersion are rendered compatible to each other to allow for coping with an ultra high speed signal. The square-lattice photonic crystal may also be a photonic crystal of an arbitrary two-dimensional Bravais lattice.
It is another object of the present invention to reduce the size of the optical delay element to improve the integration degree of the optical integrated circuit.
It is a further object of the present invention to reduce the effect of reflection in compensating for the group velocity dispersion.
In a first aspect of the present invention, there is provided an optical control device including a plurality of line-defect waveguides provided in a photonic crystal, in which each line-defect waveguide includes a multiplicity of dielectric pillars with a finite height arranged at lattice points of a two-dimensional Bravais lattice. The optical control device comprises: a first line-defect waveguide; a second line-defect waveguide provided with the dielectric pillars having a thickness (cross-sectional area) different from that of the dielectric pillars of the first line-defect waveguide; and a third line-defect waveguide. The third line-defect waveguide is arranged between the first and second line-defect waveguides and provided with the dielectric pillars whose cross sectional areas are gradually varied from those of the dielectric pillars of the first line-defect waveguide to those of the dielectric pillars of the second line-defect waveguide along a wave guiding direction.
In a second aspect of the present invention, there is provided an optical control device including a plurality of line-defect waveguides provided in a photonic crystal, in which each line-defect waveguide includes a multiplicity of dielectric pillars with a finite height arranged at lattice points of a two-dimensional Bravais lattice. The optical control device comprises: a first line-defect waveguide; a second line-defect waveguide provided with dielectric pillars having cross-sectional shapes different from those of the dielectric pillars of the first line-defect waveguide; and a third line-defect waveguide. The third line-defect waveguide is arranged between the first and second line-defect waveguides and provided with the dielectric pillars whose cross-sectional shapes are gradually varied from those of the dielectric pillars of the first line-defect waveguide to those of the dielectric pillars of the second line-defect waveguide in the course from the first line-defect waveguide towards the second line-defect waveguide along the wave guiding direction.
In a third aspect of the present invention, there is provided an optical control device including a plurality of line-defect waveguides provided in a photonic crystal, in which each line-defect waveguide including a multiplicity of dielectric pillars with a finite height arranged at lattice points of a two-dimensional Bravais lattice. The optical control device comprises: a first line-defect waveguide; a second line-defect waveguide provided with the dielectric pillars that have lattice-point intervals (that is, local lattice constants) different from those of the dielectric pillars of the first line-defect waveguide; and a third line-defect waveguide. The third line-defect waveguide is arranged between the first and second line-defect waveguides and provided with dielectric pillars whose lattice-point intervals are gradually varied from those of the dielectric pillars of the first line-defect waveguide to those of the dielectric pillars of the second line-defect waveguide in the course from the first line-defect waveguide towards the second line-defect waveguide, along the wave guiding direction.
In a first formulation of the optical control device according to the first aspect, a further line-defect waveguide, whose dielectric pillars have cross-sectional areas gradually varied along the wave guiding direction, is provided at one end or at each of both ends along the wave guiding direction of the optical control device including the first to third line-defect waveguides.
In a second formulation of the optical control device according to the first aspect, the length along the wave guiding direction of the third line-defect waveguide is not less than five times a lattice pitch (lattice constant).
In a third formulation of the optical control device according to the first aspect, the dielectric pillars contained in the line defect of the first line-defect waveguide and the dielectric pillars not contained in the line defect are thicker (larger in cross section) than the that of the dielectric pillars contained in a line defect of the second line-defect waveguide and dielectric pillars not contained in the line defect. Or, the dielectric pillars contained in the line defect of the second line-defect waveguide and the dielectric pillars not contained in the line defect are thicker than that of the dielectric pillars contained in a line defect of the first line-defect waveguide and dielectric pillars not contained in the line defect.
In a fourth aspect of the optical control device according to the first aspect, thicknesses (cross-sectional areas) of the dielectric pillars contained in the line defect of the first and second line-defect waveguides differ only with respect to those of the dielectric pillars contained in the line defect of the first and second line-defect waveguides, and the thicknesses (cross-sectional areas) of dielectric pillars not contained in the line defect(s) of the first and second line-defect waveguides are equal.
In a fifth formulation of the optical control device according to the first aspect, thicknesses (cross-sectional areas) of the dielectric pillars not contained in the line defect of the first and second line-defect waveguide differ only with respect to those of the dielectric pillars not contained in the line defect(s) of the first and second line-defect waveguides, and the thicknesses (cross-sectional areas) of the dielectric pillars contained in the line defect(s) of the first and second line-defect waveguides are equal.
In a sixth formulation of the optical control device according to the first aspect, the thicknesses (cross-sectional areas) of the dielectric pillars contained in the line defects of the first and second line-defect waveguides gradually change in opposite directions between the dielectric pillars not contained in the line defect of the first and second line-defect waveguides and the dielectric pillars contained in the line defect of the first and second line-defect waveguides.
In a seventh formulation of the optical control device according to the second aspect, a further line-defect waveguide, whose dielectric pillars have cross-sectional shapes gradually varied along the wave guiding direction, is provided at one end or at each of both ends along the wave guiding direction of the optical control device including the first to third line-defect waveguides.
In an eighth formulation of the optical control device according to the second aspect, the length along the wave guiding direction of the third line-defect waveguide is not less than five lattice periods.
In a ninth formulation of the optical control device according to the second aspect, the cross-sectional shapes of the dielectric pillars contained in the line defect of the first and second line-defect waveguides differ only with respect to those of the dielectric pillars contained in the line defect of the first and second line-defect waveguides, and the cross-sectional shapes of dielectric pillars not contained in the line defect of the first and second line-defect waveguides are equal.
In a tenth formulation of the optical control device according to the second aspect, the cross-sectional shapes of the dielectric pillars not contained in the line defect of the first and second line-defect waveguides differ only with respect to those of dielectric pillars not contained in the line defect of the first and second line-defect waveguides, and the cross-sectional shapes of the dielectric pillars contained in the line defects of the first and second line-defect waveguides are equal.
In an eleventh formulation of the optical control device according to the second aspect, the cross-sectional shapes of the dielectric pillars contained in the line defects in the first and second line-defect waveguides are equal to the cross-sectional shapes of the dielectric pillars not contained in the line defects in the first and second line-defect waveguides.
In a twelfth formulation of the optical control device according to the second aspect, in at least one of the first and second line-defect waveguides, the cross-sectional shapes of the dielectric pillars contained in the line defects are different from the cross-sectional shapes of dielectric pillars not contained in the line defects.
In a thirteenth formulation of the optical control device according to the third aspect, a further line-defect waveguide, whose lattice-point intervals are gradually varied along the wave guiding direction, is provided at one end or at each of both ends along the wave guiding direction of the optical control device including the first to third line-defect waveguides.
In a fourteenth formulation of the optical control device according to the third aspect, the length along the wave guiding direction of the third line-defect waveguide is not less than five times a lattice period (pitch).
In a fifteenth formulation of the optical control device according to the third aspect, the lattice-point intervals of the dielectric pillars contained in the line defect of the first and second line-defect waveguides differ only with respect to those of the dielectric pillars contained in the line defect of the first and second line-defect waveguides, and the lattice-point intervals of dielectric pillars not contained in the line defect of the first and second line-defect waveguides are equal.
In a sixteenth formulation of the optical control device according to the third aspect, only lattice-point intervals of the dielectric pillars not contained in the line defect of said first and second line-defect waveguides differ, and wherein the lattice point intervals of the dielectric pillars contained in the line defect(s) of said first and second line-defect waveguides are equal.
In a seventeenth formulation of the optical control device according to the third aspect, in the first and second line-defect waveguides, the lattice-point intervals of the dielectric pillars contained in the line defects are equal to the lattice-point intervals of the dielectric pillars not contained in the line defects.
In an eighteenth formulation of the optical control device according to the third aspect, in at least one of the first and second line waveguides, lattice-point intervals of the dielectric pillars contained in the line defect are different from lattice-point intervals of the dielectric pillars not contained in the line defects.
In a nineteenth formulation of the optical control device according to any of the first to third aspects, the dielectric pillars are formed of a material higher in dielectric constant than a background medium.
In a twentieth formulation of the optical control device according to any of the first to third aspects, the dielectric pillar(s) is(are) a hole(s) provided in a high-refractive-index material.
In a twenty-first formulation of the optical control device according to any one of the first to third aspects, the two-dimensional Bravais lattice is a square lattice.
With the optical control device of the present invention, group velocity dispersion can be compensated while delay is kept long for application as an optical delay element. Hence, the delay element of the present invention may be applied even for an ultra-high speed signal spanning non-negligible range of wavelength. It is observed that, since the adverse effect of the waveform distortion due to group velocity dispersion can be suppressed, the optical control device (optical delay element) can be extended in its length along the wave guiding direction to significantly protract the delay time.
Also, a wavelength range in which group velocity was smaller but, originally, group velocity dispersion had been too large can be made available, so that optical delay can be achieved with a device with a waveguide shorter than those in conventional devices. Hence, the optical delay element can be reduced in size.
Additionally, with the optical control device of the present invention, reflection of light may be suppressed by interconnecting two different material types differing in the group velocity dispersion through a line-defect waveguide provided with a gradually varying structure.